If you're wondering how many people you'll need to make a third-person online strategy game for Facebook, then I don't have experience with Facebook as a platform - my development thus far is purely PC.
The general answer would depend on the size of the game - how much content the game has, how long it takes each artist to make a piece of content, and how many years you want to work on it. When you actually get into development though, there are all kinds of hidden challenges that show up.
You'll need people to:
...write the logic of the game in computer code
...sketch and maybe color concept art
...make the 3D models from that concept art
...rig the 3D models for animation
...create textures for the 3D models
...compose music
...make sound effect
...draw concepts of the game world
...craft the game world in your editor
And probably more things I haven't thought of.
Now, some people can do multiple of those tasks, but if you have alot of content that needs to be made, you'll need multiple people working in some of those categories. For example, maybe your composer can compose the music and do the sound effects (if he's skilled in both areas)... but if you need alot of music, he won't have time to do both.
Maybe your 3D modeller is skilled enough to sketch the concepts, shape the 3D model, rig the model, and create textures for it. But maybe your world is so large that you need half a dozen map makers.
It really depends on the nature and scale of the game you want to make. Some games are plot-heavy with only a little art and content, some games are art-heavy or content-heavy with little plot; some games have huge worlds, some games have compact worlds.
Most open-world games, and especially online ones, fall into the "lots of content and art" with "huge worlds" category.
If this is your first game, you shouldn't start out trying to paint the Mona Lisa. The best way to paint the Mona Lisa is to first learn how to straight lines with a pencil on a piece of paper. Eventually you'll become skilled enough to paint masterpieces, but trying to paint amazing artwork - even smaller amazing artworks - without first learning the intermediate steps, often leads to discouragement and disappointment. Just ask my stick-figure art.
There's a reason why armies train their soldiers before dropping them on a battlefield - they won't learn on the battlefield, they'll just die there. They need to learn before they encounter the enemy, and then their training gets refined and grows on the battlefield; they have to have the solid foundation of training beforehand.
Videogames are easy to play, but they are very complex beasts to make. So, game developers really do start very very small with very simple games and then work their way upwards one completed game at a time - each game getting slightly larger and slightly more complex than the previous one.
Again, I'm working off of the assumption that this is your first game, and the first game of your friends. If this is not your first game, and you've already taken the steps of making smaller games, and are now actually trained and ready to make a larger game, awesome!
It's just... we see a continual stream of people who find a sword on the ground and decide to run off and kill the dragon. We try to shout, "Learn to use the sword first! Train to become a warrior! Fight straw dummies before attacking a dragon!", but they ignore us and get eaten. The end.
Not that I've slain the dragon myself - I've merely survived by not slaying the dragon, instead practicing on chickens. And even then, I sometimes bite off more than I can chew.